1. Technical Field
The present invention relates to flash memory and more particularly, to a flash memory device and writing method thereof.
Semiconductor memories are mostly classified into volatile and nonvolatile types. Volatile memory devices are faster in reading and writing data, but loose their data if there is loss of power supply. Nonvolatile memory devices retain their data even when there is loss of power supply. For that reason, nonvolatile memory devices are used to store data or information to be retained regardless of power supply. In many kinds of nonvolatile memory devices, flash memories or flash EEPROMs (electrically erasable and programmable read-only memories) are well suited for large-capacity storage units because of high integration density.
Flash memories may be generally differentiated into NAND and NOR types. NAND flash memories conduct writing and reading operations in the mechanism of Fowler-Nordheim (F-N) tunneling effect, while NOR flash memories conduct writing operations by hot electron injection and erasing operations by F-N tunneling effect. NOR flash memories, as code storage, are mostly employed in mobile telephone terminals desiring fast data processing because they are operable in high frequency.
NOR flash memories are divided into stacked and split gate types in accordance with a gate structure of the memory cells.
FIG. 1 is a section of a NOR flash memory cell 10 with a stacked gate structure.
Referring to FIG. 1, the memory cell 10 includes N+ source and drain regions 13 and 14 formed in a P-type substrate 19, an insulation film 15 with a thickness less than 100 Å, a floating gate (FG) 16, an intergate insulation film (e.g., ONO film) 17, and a control gate (CG) 18, which are formed on a channel region in sequence. The source region 13, the drain region 14, and the control gate 18 are connected to a source line SL, a bit line BL, and a word line WL, respectively.
During a writing operation, the source line SL and the substrate 19 are grounded in the NOR flash memory cell 10 with the stacked gate structure. The word line WL is supplied with a word line voltage of about 10V, while the bit line BL is supplied with a bit line voltage of about 5V. Under this bias condition, electrons are injected into the floating gate 16 from a channel region adjacent to the drain region 14. This mechanism is referred to as “hot electron injection.”
In general, when a voltage of about 5V is being applied to the drain region 14 of the memory cell during a writing operation, a cell current about 200μA flows through the channel region from the drain region 14 to the grounded source region 13. For instance, if data bits in the unit of a byte or a word are written at a time, the byte writing operation uses a current of 1.6 mA (200 μA×8) at maximum while the byte writing operation uses a current of 3.2 mA (200 μA×16) at maximum.
FIG. 2 is a section of a NOR flash memory cell 20 with a split gate structure.
Referring to FIG. 2, the memory cell 20 includes N+ source and drain regions 23 and 24 formed in an N-type substrate 29, and a gate oxide film 25 and a floating gate (FG) 26 which are formed on a channel region. On the floating gate 26, a tunnel oxide film 27 is formed. A control gate (CG) 28 is formed on the tunnel oxide film 27 and the gate oxide film 25 that is placed on the channel region. The source region 23, the drain region 24, and the control gate 28 are connected to a source line SL, a bit line BL, and a word line WL, respectively.
Generally, during a writing operation, the source line SL of the NOR flash memory cell 10 with the stacked gate structure is supplied with a source line voltage VSL of about 9V, while the word line WL is supplied with a word line voltage VWL of about 10V. The bit line BL is supplied with a bit line voltage VBL of about 0.5V to 2V. Under this bias condition, electrons are injected into the floating gate 26 from a channel region adjacent to the source region 24 in the mechanism of hot electron injection. In the NOR flash memory cell 20 of a split gate structure, a writing voltage of about 9V is applied to the source line SL during a writing operation.
As aforementioned, since a high voltage of about 4V through 6V is applied to the drain region of the stacked-gate NOR flash memory cell 10 during a writing operation, a writing current of a predetermined level is used. A high voltage used in the NOR flash memory during a writing operation is generated from a charge pump embedded therein. Therefore, the number of memory cells to be written at a time for reducing current consumption is confined within a restrictive number (e.g., 4, 8, 16, or 32 bits). This restriction on the number of bits to he written is more applicable to the split-gate NOR flash memory rather than the stacked-gate NOR flash memory.
In addition, the restriction of the writable bit number is also applied to a test operation conducted during production. Generally, as production quality test is carried out to check functions of products themselves, and there is generally less current consumed during testing than in normal operation. Accordingly, given the reduced current consumption, the length of time needed to complete the testing is relatively long.